This invention relates to a magnetic recording medium in a magnetic memory unit, such as a magnetic disk unit, magnetic tape unit or magnetic card unit, and a high-capacity magnetic memory unit using same.
To achieve a high-capacity of a magnetic memory unit requires high recording density and high output to noise ratio. For this reason, conventional studies have focused on perpendicular magnetic recording methods using a perpendicular magnetic film and longitudinal magnetic recording methods using a thin film longitudinal medium. The historical backgrounds of these methods will be briefly discussed hereinbelow.
The perpendicular magnetic recording medium has a small interbit demagnetization field and, in principle, it is hence suitable for achievement of high recording density. However, the perpendicular magnetic recording medium has a defect that its output is small. In order to overcome this defect, various composite magnetic films have been heretofore proposed. For example, a perpendicular magnetic recording method has been assiduously studied to achieve high recording density. As a perpendicular magnetic recording medium which is highly producible and has good perpendicular orientation, Japanese Patent Kokai (Laid-open) No. 9821/1986 proposes a composite perpendicular magnetic film in which a Co-O or Co-Ni-O film having a thickness of 0.14-0.175 .mu.m is further perpendicularly oriented on a CoCr film having perpendicular magnetic anisotropy. Further, composite magnetic recording media comprising a perpendicular magnetic thin film formed on a nonmagnetic substrate through a soft magnetic film with high permeability are also proposed in order to enhance read and write efficiency in high recording density and obtain high read output, as are discussed in Japanese Patent Kokai (Laid-open) Nos. 78403/1977, 5184/1979, etc. In general, the media are formed by an evaporation method, sputtering method, plating method, ion beam sputtering method, or the like. When a magnetic head for perpendicular magnetic recording is used with these composite magnetic recording media to carry out read and write in a state that the head is in contact, very high recording density is achieved. It is however said that when the magnetic head flies apart from a medium surface, the recording density in this method adversely decreases as compared with a conventional longitudinal recording method. Further, another problem of this method is that when a high permeable, soft magnetic layer is used as an underlayer, the perpendicular orientation of a perpendicular magnetic film formed thereon is degraded. In order to overcome this problem, Japanese Patent Kokai (Laid-open) No. 131228/1986 proposes a perpendicular magnetic recording medium in which a perpendicular magnetic film of Co-Cr, etc., is formed on a high permeable, soft magnetic underlayer of Ni-Fe, etc., through a nonmagnetic layer of Ti, etc., to prevent a magnetic interaction between this high permeable, soft magnetic layer and the perpendicular magnetic film, and a high permeable, soft magnetic layer of Ni-Fe is further formed thereon.
Further, Japanese Patent Kokai (Laid-open) No. 222022/1986, etc., propose perpendicular magnetic recording media in which a CoCr layer having a large saturation magnetization and magnetically perpendicular isotropy is formed on a magnetically isotropic low coercivity layer of CoCrNb or CoCrTa. In this case, in principle, CoCrNb or CoCrTa corresponds to what is called a high permeable, soft magnetic layer (high permeable underlayer). In these conventional composite magnetic recording media constituted of a perpendicular magnetic layer and a high permeable, soft magnetic layer, the soft magnetic layer merely plays auxiliary roles of efficiently directing the recording magnetic field of a magnetic head to the perpendicular magnetic layer and stabilizing a recording magnetization. That is, these two magnetic layers work, magnetically independently of each other. In such a constitution, since the magnetic films constituting a composite magnetic thin film have only a weak magnetic interaction, an in-plane magnetization curve is generally a simple superimpose of the in-plane magnetization curve of each of the layers. For this reason, the magnetization curve exhibits a magnetization jump and forms a so-called snake form, and any of the layers does not undergo magnetic reversal with the same in-plane coercive force. It is said that the snake-like magnetization curve having a plurality of coercivities is preferable for achievement of high recording density.
However, in order to enhance read and write efficiency, the perpendicular magnetic recording medium of any method requires the use of a main pole type magnetic head constituting a magnetic circuit with the high permeable underlayer, and it is thought that practical performance can be improved only when not only the recording method but also the entire system are changed, as is discussed in Japanese Patent Kokai (Laid-open) No. 261025/1985. In fact, when a perpendicular magnetic recording medium having this constitution is used to read and write with a conventional ring type magnetic head, the following problem takes place.
When the above composite perpendicular film medium, in which a perpendicular magnetic layer is formed on a high permeable underlayer, is used to read and write with a conventional ring type magnetic head for longitudinal recording, unlike a case of reading and writing with a main pole type perpendicular magnetic recording head, there is a problem that a very large, spike-like Barkhousen noise is generated from the high permeable underlayer, and even when the spike-like Barkhousen noise is decreased, the absolute value of media noise is large as is discussed in J. Appl. Physics 57 (1985), pages 3,964 to 3,966. It is also considered that like problems occur in a perpendicular multilayered film medium of Japanese Patent Kokai (Laid-open) No. 261025/1985 in which a high permeable, soft magnetic layer is formed on a perpendicular magnetic layer. The reasons therefore are that since the coercivity of the high permeable film is, in principle, as low as several Oe to 20 Oe and for some other reasons, a strong recording head field hardly reaches the CoCr perpendicular magnetic layer, and further that an output waveform is extremely deformed for the following reasons. That is, the perpendicular magnetic layer and the high permeable layer are magnetically weakly connected, and each of the layers responds to a recording field separately and is likely to undergo magnetization reversal separately. Further, the high permeable layer is formed under the perpendicular magnetic layer through a nonmagnetic intermediate layer having a thickness of about 200 .ANG. in order to prevent an magnetic interaction between the high permeable layer and the perpendicular magnetic layer, and hence each of the layers undergoes magnetization reversal independently of the other for a recording field. Such problems do not take place in a main pole type magnetic head.
As discussed above, the perpendicular magnetic recording method essentially has markedly superior characteristics to the longitudinal magnetic recording method if a medium is used in a state that a head is in contact with the medium.
However, a study has been again under way especially recently of improvement in the longitudinal magnetic recording method using a conventional ring type magnetic head in a head-flying state o from the viewpoint of its entire system. As a thin film medium for longitudinal magnetic recording, it is necessary to develop a magnetic thin film material having high corrosion resistance, high saturation magnetization and high coercivity, and the following composite media have been proposed.
That is, as is discussed in Japanese Patent Kokai (Laid-open) Nos. 261026/1985, 261025/1985, etc., as a combination of a perpendicular medium and a longitudinal medium, there are proposals of a composite magnetic recording media and magnetic recording method in which a magnetic thin film of Co-Ni, etc., with high in-plane coercivity or a magnetic thin film of .gamma.-Fe.sub.2 O.sub.3, etc., with isotropic coercivity is formed on a perpendicularly magnetizable film of Co-Cr, etc., whereby read and write in high recording density are carried out by using a conventional ring head. As a medium which is usable according to the same method as above and further has a different constitution, Japanese Patent Kokai (Laid-open) No. 34721/1986 proposes a double-layer medium in which a Co-Cr perpendicular magnetic layer having a perpendicular coercivity of 400 Oe or more is formed on a high-coercivity Co-Pt longitudinal recording layer having an in-plane coercivity of 300 Oe.
Further, Japanese Patent Kokai (Laid-open) No. 256217/1987 proposes a highly corrosion-resistant, longitudinal composite magnetic recording medium in which a CoCr-based alloy layer is formed on a Co-based magnetic alloy layer of CoNi, CoPt, etc., with in-plane anisotropy. In this medium, the surface layer portion of the Co-based magnetic alloy is partially replaced with the CoCr-based alloy having higher corrosion resistance than the Co-based magnetic alloy.